1. Technical Field
The present invention relates to an image forming method and an image forming apparatus used in the field of copy machines and printers using an electrophotographic process.
2. Description of Related Art
Recently, organic photoconductors have been widely used for electrophotographic photoconductors. The organic photoconductor is more advantageous than other photoconductors in that it is easy to develop materials corresponding to various exposure light source from visible light to infrared light, it is possible to select materials with no environmental pollution and production costs are inexpensive. However, there are drawbacks, e.g., weak mechanical strength and chemical durability, deterioration of electrostatic property of the photoconductor when numerous sheets are printed, occurrence of surface scarring, and the like.
That is, since electric and mechanical external forces are directly added to the surface of the organic photoconductor (hereinafter, also simply referred to as a photoconductor) by a charging unit, a development unit, a transfer unit and a cleaning unit and the like, durability against them is required.
Specifically, the durability is required against the occurrence of abrasion and scarring on the photoconductor surface due to friction, surface deterioration by active oxygen such as ozone, nitrogen oxide and the like produced at corona charging.
In order to solve problems of the mechanical and chemical durability as the above, a construction where a layer structure is a laminated construction of a charge generation layer and a charge transport layer, the charge transport layer of a protection layer is a uniform layer which has high strength and is difficult to permeate active gases, and a thickness of the charge transport layer is more than 20 μm has been frequently employed as the organic photoconductor.
Also, as the other approach, a technology where a protection layer with high strength is placed on the surface of photoconductor or the like has been studied. For example, in JP-Tokukaihei-6-118681A, it has been reported to use a hardened silicone resin as the protection layer of the photoconductor. However, in a method of thickening the film of the charge transport layer and a method of placing the protection layer with high strength as described above, there is problematic in that carriers produced in the charge generation layer diffuse in a crosswise direction before reaching the surface, resulting in problems in sharpness and the like. In the field of digital copy machines, demands for high image quality have been increased and the image formation with high resolution has been studied, but no favorable electrostatic latent image can be obtained by the layer structure and the protection layer where it is easy to result in diffusion of the carriers in this way.
In order to obtain electrophotographic images with high image quality, it is necessary to faithfully reproduce image information as the electrostatic latent image and sharply form potential contrast of exposed/unexposed portions on the organic photoconductor, and therefor, it is important to inhibit the diffusion of carrier until the produced carrier reaches the surface charge. For latent image deterioration of images with high density, it has been reported in The Imaging Society of Japan, Vol. 38, No. 4, pp. 296 that an effect of the diffusion to the electrostatic latent image can not be negligible when D/μ which is a ratio of a diffusion constant (D) in the charge transport layer to drift mobility (μ) becomes large, and that the latent image deterioration becomes large when the thickness of the charge transport layer becomes large.
Also, it has been reported that the photoconductor with thin film thickness is effective for toner scattering prevention of toner images (see JP-Tokukai-2001-209206A), and that the photoconductor with thin layer and large electrostatic capacity can form electrophotographic images with high quality (see JP-Tokukai-2001-312082A). Similarly, the organic photoconductor where the thickness of the charge transport layer is thinned and the diffusion of electrostatic latent image is prevented has been already proposed (see JP-Tokukaihei-5-119503A).
However, when the image formation is performed actually using these proposed organic photoconductors with thin layer for an electrophotographic image forming apparatus, image unevenness due to mechanical attrition unevenness becomes easily impressive as compared to the earlier organic photoconductor with film thickness of 20 μm or more, and it is difficult to maintain image quality over a long time period. This cause is attributed to that the smaller the film thickness of a photosensitive layer is, the larger the reduction of potential retention ability (ΔV1>ΔV2) is in the case of the same attrition amount as shown in FIG. 1 (figure showing dependency of the potential retention ability reduction on the thickness).
As a result, when the organic photoconductor with thin film is used, the image unevenness due to the attrition unevenness of the organic photoconductor easily occurs.
Also when using the organic photoconductor with thin film and using a developer using toner with small particle size for obtaining the high image quality, toner scatterings in text easily occurs at a portion where the attrition unevenness occurs in the organic photoconductor. This is attributed to the presence of a toner component which easily flies in all directions in concavoconvex areas of the attrition unevenness of the organic photoconductor surface.